**2. Chirality**

A chiral structure is non-superimposable on its mirror image. Pasteur reported in 1984 the concept of the molecular Chirality based on the distinction between the configurational isomers of molecules. Configurational isomers are compounds with the same molecular formula and same groups but different configurations. Enantiomers are pairs of configurational isomers that are mirror images of each other but are non-superimposable. Diastereomers are pairs of compounds that contain more than one chiral center, not all of which are superimposable. An equimolar mixture of opposite enantiomers is called racemic mixture or a racemate. Enantiomers when exposed to polarized light behave differently and have different catalyzing properties in a chiral medium. On the other hand, the racemic mixtures have completely different properties than enantiomers. The difference in the properties between the enantiomers and racemic mixtures arise due to different molecular interactions, and different crystal structures. [30, 31]

In an enantiomer, the molecular interactions are homochiral, which are the interactions between the assemblies of molecules with same Chirality. In a racemic compound the interactions are heterochiral, where the interactions are between opposite chiral molecules. The difference between the homochiral and heterochiral interactions leads to different physical properties. Particularly in a racemic compound, because the unit cell consists of enantiomeric molecules with opposite Chirality, the properties are completely different from enantiomers. Racemic compounds are the most common compounds that occur in nature. Such racemic compounds can exist in different forms based on the intermolecular interactions in their crystals. Analysis of the crystal structures facilitates enormously our understanding of the factors that determine the various physical and chemical properties, such as the thermodynamic stability of different types of racemates. [32] The details of such analysis are beyond the scope of this chapter.

Due to the presence of various chiral compounds, it is critical to have the right nomenclature for the differentiation. The internationally accepted nomenclature for chiral molecules uses the Cahn-Ingold-Prelog (CIP) rules for sp3 carbons. The four substituents are sorted by increasing mass of the first atom attached to the asymmetric center. If two atoms are identical, the next heaviest atom one bond further away is considered and so on. For example, in the case of 2-butanol with the order OH ethyl methyl rotating clockwise will be a R-enantiomer and the mirror image of that will be a S-enantiomer. These rules allow us for the absolute configuration of any chiral compounds. Another accepted form for nomenclature is Dextro (D-) and Levo (L-), based on the optical rotation of the compound. Setting glycine apart since it is nonchiral, it must be noted that all amino acids found in proteins are L-amino acids and also have the S-configuration at the exception of cysteine whose -CH2-SH substituent precedes the carboxylate -COOH in mass making L-cysteine the R-enantiomer. It is interesting to note that the electrodeposited chiral films also follow the CIP rules. It was shown that the CuO films grown from both R versions of tartaric acid and malic acid resulted in (1-1-1) orientation on Cu(111), while the S version of each resulted in a (-111) mirror image. [33] However, there are exceptions for films grown from amino acids which will be discussed in detail.
